The present invention relates to a semiconductor device, and more particularly to the structure of a resin-sealed semiconductor device.
The reliability of a semiconductor device, especially of an insulated gate field effect IC device such as a MOS IC device, the operation of which relies upon a semiconductor substrate surface phenomenon, often is determined mainly by the surface condition of the semiconductor device. Where the semiconductor substrate is hermetically encapsulated within a ceramic casing, impurities in the atmosphere within the casing are kept at a low level and the humidity within the casing is suppressed to keep the surface condition of the substrate clear. Furthermore, in such a device, the surface of the substrate is usually coated with a silicon oxide film formed using a chemical vapor deposition (CVD) process to thus prevent penetration of impurities and thereby suppress variations in the electrical characteristics of the device.
Although this SiO.sub.2 coating is fully effective as a protective film when a ceramic casing is employed, it is known that, if a plastic or resin casing is used instead of a ceramic casing, variations in electrical characteristics may in fact occur. This is due to the fact that the casing itself contains impurities such as sodium ions (Na+) or the like (generally in an epoxy resin such ions are present in a proportion of about 100 ppm). Where the casing itself directly contacts the surface of semiconductor device elements formed in the substrate, the Na+ ions permeate through the SiO.sub.2 protective coating, resulting in variations in the electrical characteristics of the device.
The use of a plastic or resin casing thus gives rise to a problem in reliability over a long period of use; however, on the other hand, a plastic casing possesses a number of advantages such as ease of mass productivity and low cost. In order to overcome the above mentioned problem and to provide plastic casings having the same degree of reliability as ceramic casings, various surface protective films have been investigated.
In recent years, a thin film of low concentration phosphorus silicate glass (PSG) or silicon nitride (Si.sub.3 N.sub.4) has begun to be used as the surface protective film. If the thickness and the growing temperature of such thin films are appropriately selected, the film exhibits a sufficient protection against impurities such as Na+ ions or the like, even if the semiconductor element is encased in plastic or resin, so that it has become possible to obtain a highly reliable IC which does not have degraded characteristics, even if it is continuously operated at a high temperature of 125.degree. C. This protective film is similarly effective against moisture, and can prevent corrosion of aluminum wiring or interconnection layers and the accompanying generation of surface leakage current which is caused by moisture penetrating along boundaries between the plastic or resin coating and the lead electrodes.
However, it is essentially impossible even for such an excellent protective film to cover an aluminum bonding pad where a metallic fine wire (generally a gold wire of about 30 .mu.m in diameter) is bonded. Since the protective film must be removed from the upper surface of the bonding pad to form an aperture, the moisture resisting ability of the coating is reduced by the presence of this aperture.